Intra-morph body colouration may correlate with performance in the eastern red-backed salamander (Plethodon cinereus)

Extensive intraspecific phenotypic variation, such as the discrete morphs found in polymorphic species, is often linked to morph-specific ecologically relevant behaviours or physiologies. Whether within morph variation in morph-specific phenotypic traits similarly correlates with behavioural and physiological traits remains unclear. The Eastern Red-backed Salamander (Plethodon cinereus (Green, 1818)) exhibits two predominant morph types: striped (“red-backed”) and unstriped (“lead-backed”), which differ not only in body colour but also habitat and performance abilities. Whether similar correlations of body colour and performance exist within morphs is understudied. Here, we tested whether intra-morph variation in the striped morph’s dorsal colouration is linked to an ecologically relevant performance trait: sprint speed. We measured individual sprint speed from three populations and used standardized digital photography to quantify two colour parameters (intensity and redness). The relationship of intra-morph redness and sprint speed varied among populations, with one population demonstrating a significant positive relationship. The varying relationship of redness and performance may represent population-level variation in individual energetic capability, such that only some populations exceed a threshold of habitat resources where a colour-performance relationship becomes detectable. Our results show previously overlooked intra-morph variation in a widely studied polymorphic species and suggest how variation within colour may relate to fitness-relevant traits.